As phenomena go, ball lightning is one of those things which has always fascinated me. It was said that Tesla could summon a ball of light[1], no one has ever been able to reproduce that feat. That in itself has got to be pretty cool.

I like the idea of a the microwave bubble (or even just an RF bubble). We know that ionized air reflects radio waves (ask any amateur who has bounced off the ionosphere for an over the horizon signal) so I can imagine that a 'ball' of ionized air would reflect any radiowaves that were inside of it with energy leaking out in the form of light. But how to do that with Tesla era equipment? Good question.

[1] "I never saw fireballs, but as a compensation for my disappointment I succeeded later in determining the mode of their formation and producing them artificially. " -- http://www.bibliotecapleyades.net/tesla/esp_tesla_20.htm